The present invention relates to a device for dusting stripor sheet-shaped products with which a powder/air mixture is sprayed on the products and to a method for dusting such products.
Such devices and method for dusting are, for example, employed in stacking devices for printed sheets of paper in a sheet-fed offset printing press, so that for drying the printing ink the individual sheets of paper are separated from each other by means of, a powder. In the course of this the problem occurs, that the very fine-grained powder, which is applied to the sheets of paper via an air stream, is only partially deposited on the sheet of paper. Strong air eddy currents occur because of the high conveying speeds of the sheets of paperxe2x80x94apporoximately 15,000 sheets per hourxe2x80x94, so that a large fraction of the powder does not reach and remain on the sheet of paper, but adds to the contamination of the printing press, and of the ambient air. For improving the effectiveness of the dusting it was proposed in accordance with DE-OS 26 46 798 to charge the powder particles and to apply them in the charged state to the sheet of paper. A further device is known from DE-OS 29 46 754, with which the powder to be deposited on the sheet of paper is, in essentially electrically neutral form, dispensed from nozzles and is charged by means of a corona discharge prior to impact on the sheet of paper. This method has the disadvantage that, although the charged powder better adheres to the sheets of paper, it also adheres better to all parts of the press, in particular those that are metallically neutral. The parts of the press are thereby covered with a thick layer of powder in a short time.
For this reason a further device was proposed by DE-OS 33 30 665, in which the powder is electrically neutralized when leaving the nozzle. Although the electrostatic adhesion of powder to metallic parts of the press is prevented, the original problems once more occur, with the very fine-grained powder contaminating large areas of the printing press and the surroundings due to air currents.
The known devices therefore offer only insufficient solutions to the dusting problem, because, in the final analysis, only a small amount of the powder is deposited and remains on the sheet of paper and contamination of the printing press, and of the surroundings, is inadequately prevented. Based on this prior art, it is the object of the invention to provide an improved device and method for dusting moving objects, by means of which the powder can be effectively deposited on the sheets of paper, wherein a contamination of the printing press and the surroundings is reduced.
This object is achieved by a device including means for generating a magnetic field in the deposition area so that a force is exerted on the magnetized powder particle in a direction toward the product, and by a method according to which the powder particles are magnetized prior to impacting the product and are deposited on the product by means of a magnetic force acting on the magnetized powder particles.
A powder/air mixture can be sprayed in a known manner on the sheet of paper by means of the device of the invention, wherein in accordance with the invention the individual powder particles of the powder can be magnetized. Means are furthermore provided for creating a magnetic field in the deposition area of the powder, so that a force in the direction toward the sheet of paper acts on the magnetized powder particles. In this manner, the powder can be better separated from the air carrying it. The magnetic forces are greater than the electrostatic forces of known dusting devices. The known electrostatic forces assist in holding only the powder which has already been deposited on the sheet of paper. But by means of the device of the invention and the corresponding method, the not yet deposited powder can already be effectively influenced by the magnetic forces and can be better separated from the air carrying it.
The flow speed of the powder/air mixture can be increased, as a result of which the powder-carrying air flow is less sensitive to disturbances and therefore the surrounding press elements are less contaminated.
Since the magnetic force leads to better separation of the powder particles from the air carrying them as well as to a more effective deposition of the powder on the sheet of paper, the amount of powder used can be reduced.
Advantageous embodiments of the device in accordance with the invention are the subject the dependent claims.
Permanent or electromagnets can be used for generating the magnetic field. Electromagnets are preferred, since they can be switched off, for example, for cleaning the printing press, and can therefore be better cleaned of adhering powder.
In an advantageous manner the magnet is arranged in such a way that the product can be passed through between one pole and the spray nozzle, so that the sheet of paper to be dusted can be conveyed directly over one pole of the magnet, so that the strongest possible magnetic field prevails in the deposition area of the powder.
In an embodiment of the invention in which the magnet has a U-profile whose legs form poles which extend transverse to the transport direction and across the width of the product, it is assured that an even magnetic force acts on the powder over the entire width of the sheet of paper.
An embodiment of the invention comprises means including a plurality of electromagnets arranged parallel and transversely to the transport direction. This permits adaptation of the magnetic field to various paper widths by activating only the magnets covered by the sheet of paper to be dusted.
In accordance with another embodiment one pole of the magnet(s) is preferably arranged in the deposition area of the powder, since the greatest strength of the magnetic field prevails in the pole area. The other pole is preferably arranged downstream in the conveying direction, so that the non-deposited powder once more experiences a force in the direction toward the sheet of paper at that location. The efficiency of deposition is thereby increased.
The sheets of paper are transported past the spraying nozzle by conveying means. So that the conveying means do not interfere with the magnetic field, they preferably primarily consist of a non-magnetizable material, for example plastic. A gripper element, by means of which the sheet is grasped at its front end for conveying, is however made of a magnetizable material. In this manner, strong magnetic fields are generated at the front edge of the sheet, so that the deposition of powder is assisted by the magnetic force in this area, which would otherwise only be insufficiently dusted with powder by means of known devices.
The magnetizable powder can consist of covered or coated iron particles, or of powder particles having covering iron particles on their surface. Iron is ferromagnetic and therefore strongly magnetizable. The iron particles should be covered or coated, so that the iron cannot oxidize and leave unwanted spots on the sheet of paper.
The invention will be explained in detail in what follows by means of an exemplary embodiment, making reference to the drawings.